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From: RayeR <glaux AT centrum DOT cz>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: running ring0 code under DJGPP
Date: Sun, 02 Sep 2007 12:53:54 -0700
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Yeaaa!
it's definitely working! I can read/write MSRs even under stupid
EMM386 and Windows98SE. Breaking the wall :)

The problem above was caused by pushal-popal pair in asm code:
I'm not sure if it's really needed, plase inline-asm-skilled guys let
me know how to improve it

  asm __volatile__ (                   // volani CallGate
//    "pushal\n"                         // store all registers to
stack
    "cli\n"                            // disable interrupts
    "movl %0,%%ebx\n"                  // store pointer to low DWord
    "movl %1,%%edi\n"                  // store pointer to high DWord
    "movl %2,%%ecx\n"                  // store MSR index
    "lcall *%3\n"                      // far call the CallGate to
enter ring 0, require code far return
    "sti\n"                            // enable interrupts
//    "popal\n"                          // restore all registers from
stack
    :                                  // no outputs (readed data
overwrites input arguments)
    : "g" (eax), "g" (edx), "g" (ecx), "m" (ldt_callgate_ptr) //
inputs
    );

(rdmsr/wrmsr affects only EDX:EAX, using ECX as input param)

//***************** kod s opravnenim ring 0 - precte MSR dle ECX do
promennych na adresach [EBX], [EDI] (Win9x)
void ring0_rdmsr(void)                 // GCC bohuzel nepodporuje
__attribute__ ((naked)) na x86
{                                      // takze misto pointeru na
funkci pouzijeme ASM label
asm __volatile__ (
  "rdmsr_label:\n"                     // label at start of pure code
behind C-function prologue
  "rdmsr\n"                            // read CPU MSR register value
to EDX:EAX
  "movl %eax,(%ebx)\n"                 // mov dword ptr [ebx],eax
  "movl %edx,(%edi)\n"                 // mov dword ptr [edi],edx
  "lret\n"                             // far return from ring 0
  );                                   // C-function epilogue will be
never reached
}